Closed-loop hydraulic systems have been adapted for applications subject to variable loads and start/stop conditions. Closed-loop systems are very efficient and can be readily adapted so as to be, at least partially, regenerative. Smooth operation is, however, virtually impossible unless the closed-loop system is provided with a control system adapted for matching the system pressure to the load requirement. Prior art control systems have usually been electrical or electronic. Electrical and electronic control systems are, however, dependent on an electrical power supply. Additionally, the electrical and electronic systems are susceptible to damage caused by the adverse environmental conditions to which the systems are subjected. Furthermore, application of a control system on an international scale is complicated by the need to comply with the safety requirements of various countries.
Open-loop hydraulic systems have generally been preferred over closed-loop systems for applications with variable loads and stop/start conditions. An open-loop system is, generally, designed with fixed displacement pumps, fixed displacement motors or cylinders, directional control valves and counter-balance valves. Open-loop systems are not as efficient as closed-loop systems and they are usually not regenerative. Open-loop systems generally require power input to the pump in order to drive the motor with or without a load on the system and regardless of the direction of movement of the load. These open-loop systems, therefore, require larger power sources and utilize much fuel or energy due to the necessity to continuously power the pump. Large heat exchangers are also required in order to remove excessive heat from the hydraulic fluid.
A load recall valve monitors the pressure in a closed-loop hydraulic system and permits the coordination and/or control of secondary functions relative to the load on the hydraulic system. Many types of machinery; particularly, personnel handling equipment, jacking systems, heavy construction equipment, agricultural and food harvesting equipment, among others, utilize a hydrostatic transmission. Such equipment is decelerated by hydraulic braking. A mechanical brake is utilized to prevent further motion. Utilization of a load recall valve improves the performance of the apparatus when the operation requires that the motion be continued or reversed.
Proper operation of a closed-loop hydraulic system requires that the subsequent brake release occur only when the system pressure is equal to the system pressure just prior to the previous initiation of the brake setting process. Release of the brake when the system pressure equals the system pressure at the time of the previous setting of the brake assures that sufficient force is available to take-up the load without allowing the load to fall. Should the system pressure be lower than that just prior to setting of the brake, then the load will be subject to a jerking motion, or possibly, even to total free fall. Such a jerking motion or free fall is undesirable.
The prior art electrical and electronic systems utilize an electrically operated memory to store the system pressure just prior to the setting of the brake. Utilization of a memory permits coordination between the brake system and the hydraulic system and assures that sufficient lifting force is available when the brake is released.